No marriage may be contracted without payment of a dowry
The Synopsis and Codification, section IV.C.1.j.i.-v., summarizes the main provisions concerning the dowry. These provisions have their antecedents in the Bayán.
The dowry is to be paid by the bridegroom to the bride. It is fixed at 19 mithqáls of pure gold for city-dwellers, and 19 mithqáls of silver for village-dwellers (see note notes.php#94">94). Bahá’u’lláh indicates that, if, at the time of the wedding, the bridegroom is unable to pay the dowry in full, it is permissible for him to issue a promissory note to the bride (Q&A 39).
With the Revelation of Bahá’u’lláh many familiar concepts, customs and institutions are redefined and take on new meaning. One of these is the dowry. The institution of dowry is a very ancient practice in many cultures and takes many forms. In some countries it is a payment made by the parents of the bride to the bridegroom; in others it is a payment made by the bridegroom to the parents of the bride, called a "bride-price". In both such cases the amount is often quite considerable. The law of Bahá’u’lláh abolishes all such variants and converts the dowry into a symbolic act whereby the bridegroom presents a gift of a certain limited value to the bride.

Question: In connection with the dowry, what if the bridegroom cannot pay this sum in full, but instead were to formally deliver a promissory note to his bride at the time of the wedding ceremony, on the understanding that he will honour it when he is able to do so?
Answer: Permission to adopt this practice hath been granted by the Source of Authority.